When Kevin Wells’ private jet lost its GPS reception after taking off from Hayward Executive Airport in California’s Bay Area in February 2019, he whipped out his phone and starting filming the instrument panel. For a few seconds, the signals from over a dozen GPS satellites can be seen to blink away, before slowly returning as Wells continues his ascent. Wells was able to continue his flight safely, but may have accidentally flown too high, into commercial airspace near Oakland.

Wells was quick to film the incident because this was not the first time he had suffered GPS problems. Less than a month earlier, his Cessna had been hit by a similar outage, at almost the identical location. “When I asked the Hayward Tower about the loss of signal, they did not know of [it],” he wrote later in a report to NASA’s Aviation Safety Reporting System.

“It wasn’t a big event for me because I came out of the clouds at the beginning of the second event,” says Wells of the 2019 interference.

Wells had fallen victim to a GPS interference event, where rogue or malicious signals drown out the faint signals from navigation satellites in orbit. Such events, often linked to U.S. military tests, can cause dangerous situations and near-misses.  A recent Spectrum investigation discovered that they are far more prevalent, particularly in the western United States, than had previously been thought.

Luckily, Wells was in a position to do something about it. As Executive Director of Stanford Institute for Theoretical Physics, Wells knew many of the university’s researchers. He took his video to the GPS Lab at Stanford Engineering, where Professor Todd Walter was already studying the problem of GPS jamming.

The Federal Aviation Administration (FAA) and the Federal Communications Commission (FCC) do have procedures for finding people deliberately or accidentally interfering with GPS signals. When pilots reported mysterious, sporadic GPS jamming near Wilmington Airport in North Carolina, the FAA eventually identified a poorly designed antenna on a utility company’s wireless control system. “But this took many weeks, maybe months,” Walter tells Spectrum. “Our goal is to track down the culprit in days.”

Walter’s team was working on a drone that would autonomously sniff out local signals in the GPS band, without having to rely on GPS for its own navigation. “But we didn’t have permission to fly drones in Hayward’s airspace and it wasn’t quite at the point where we could just launch it and seek out the source of the interference,” says Walter.

Instead, Walter had a different idea: Why not use the GPS receivers in other aircraft to crowdsource a solution? All modern planes carry ADS-B transponders—devices that continually broadcast their GPS location, speed and heading to aid air traffic control and avoid potential collisions. These ADS-B signals are collected by nearby aircraft but also by many terrestrial sensors, including a network of open-access receivers organized by OpenSky, a Swiss nonprofit.

With OpenSky’s data in hand, the Stanford researchers’ first task was accurately identifying interference events. They found that the vast majority of times that ADS-B receivers lost data had nothing to do with interference. Some receivers were unreliable, others were obscured from planes overhead by buildings or trees.

Being able to link the loss of Wells’ GPS signals with data from the OpenSky database was an important step in characterizing genuine jamming. Integrity and accuracy indicators built into the ADS-B data stream also helped the researchers. “I think certain interference events have a characteristic that we can now recognize,” says Walter. “But I’d be concerned that there would be other interference events that we don’t have the pattern for. We need more resources and more data.”

For the Hayward incidents, Walter’s team managed to identify 17 days of potential jamming between January and March 2019. Of the 265 aircraft that flew near Hayward during that time, the ADS-B data showed that 25 had probably experienced GPS interference. This intermittent jamming was not enough to narrow down the source of the signals, says Walter: “We can say it’s in this region of Hayward but you don’t want to go searching ten city blocks. We want to localize it to a house or building.”

The FAA eventually issued a warning to pilots flying in and out of Hayward and much of the southern Bay Area, and the interference seemed to quiet down. However, Walter recently looked at 2020 ADS-B data near Hayward and found 13 more potentially jammed flights.

Walter now hopes to expand on its Hayward study by tapping into the FAA’s own network of ADS-B receivers to help uncover more interference signals hidden in the data.

Leveraging ADS-B signals is not the way researchers can troubleshoot GPS reception. Earlier this month, John Stader and Sanjeev Gunawardena at the U.S. Air Force Institute of Technology presented a paper at the Institute of Navigation’s PTTI 2021 conference detailing an alternative interference detection system.

The AFIT system also uses free and open-source data, but this time from a network of continuously-operating terrestrial GPS receivers across the globe. These provide high-quality data on GPS signals in their vicinity, which the AFIT researchers then mined to identify interference events. The AFIT team was able to identify 30 possible jamming events in one 24-hour period, with detection within a matter of minutes of each jamming incident occurring. “The end goal of this research effort is to create a worldwide, automated system for detecting interference events using publicly available data,” wrote the authors.

A small system like this is already up and running around Madrid Airport in Spain, where 11 GPS receivers constantly monitor for jamming or accidental interference. “This is costly and takes a while to install,” says Walter, “I feel like major airports probably do want something like that to protect their airspace, but it’s never going to be possible for smaller airports like Hayward.”

Another problem facing both these systems is the sparse distribution of sensors, says Todd Humphreys, Director of the Radionavigation Laboratory at UT Austin: “There are fewer than 3000 GPS reference stations with publicly-accessible data across the globe; these can be separated by hundreds of miles. Likewise, global coverage by ships and planes is still sparse enough to make detection challenging, and localization nearly impossible, except around ports and airports.”

Humphreys favors using other satellites to monitor the GPS satellites, and published a paper last year that zeroed in on a GPS jamming system in Syria. Aireon is already monitoring ADS-B signals worldwide from Iridium’s medium earth orbit satellites, while HawkEye 360 is building out its own radio-frequency sensing satellites in low earth orbit (LEO).

“That’s very exciting,” says Walter. “If you have dedicated equipment on these LEO satellites, that can be very powerful and maybe in the long term much lower cost than a whole bunch of terrestrial sensors.”

Until then, pilots like Kevin Wells will having to keep their wits about them as they navigate areas prone to GPS interference. The source of the jamming near Hayward Airport was never identified.

Source: IEEE Spectrum Telecom Channel